This invention relates to packing elements use in mass transfer and direct heat transfer applications and specifically to packing elements of a generally cylindrical shape, such as Lessing rings.
Such elements are used in reactors where liquids and gases are contacted so as to bring about a reaction, heat transfer, solution or dissolution operations by intimately contacting two flowing fluids. As a consequence they have shapes designed to maximize surface area while retaining dimensional stability such they do not collapse or become crushed during use. One means of achieving this end is to form the rings from cylindrical metal blanks, and providing internal structure by a variety of suitable techniques.
It is however desirable to have a structure that is easily produced to a standard design in large volumes. To satisfy these requirements the elements are often stamped out of a flat metal plate intended to be bent into a cylindrical shape. One type of ring of this type is the so-called Lessing ring in which a metal strip is bent into a cylindrical shape and one end of the strip is bent inwardly along a chord of the cross-section of the cylinder.
In order for the ring to maintain its shape after it has been formed it must be made from a material with properties of high malleability, high elongation, (that is greater than about 30%) and relatively low yield stress and/or high modulus of elasticity. Examples of such materials include mild carbon steel, 304 stainless steel and the like. Elongation, Yield Stress and Modulus of Elasticity are all measured by the technique shown in ASTM E8. In the context of this invention a "high" yield stress is understood to be a value of 290 mPa or more, while a "low" yield stress is 260 mPa or less. Likewise a "high" modulus of elasticity is one with a value of 170 GPa or more and a "low" value is below 170 GPa.
If an attempt is made to make a ring from a low malleability/low elongation (that is, less than about 20%) and high yield stress material, the ring will spring back after formation leaving a large gap in the ring. Examples of such materials are zirconium and titanium.
In order to retain the closed ring structure it is possible to use thicker gauge metal strip. This however becomes very costly and adds large weight penalties to the product. Alternatively it is necessary to develop a design with a mechanical closure. Such a design is taught in U.S. Pat. No. 5,304,328.
One mechanical solution has been to use a pair of bridging members to hold two cooperating semicylindrical pieces together as is described in U.S. Pat. No. 4,197,264. This has a significant disadvantage in that the production process requires the combination of four separate components to make the final product.
As indicated above, another solution proposed in U.S. Pat. No. 5,304,328 provides a metal strip having first and second end zones adjacent the ends thereof, adapted to be bent into a generally cylindrical form to produce a packing element; wherein the first end zone is provided with a bend such that, when the element is formed, the end of the strip is within the cylinder and the first end zone is further provided, in the vicinity of the bend, with one or more slots; and in which the second end zone is provided with one or more tabs projecting from the end of the strip and adapted to engage with the slots in the first end zone so as to lock the ends together when the strip is formed into a cylindrical packing element.
While effective for many applications the above design is rather difficult to make consistently.
The present invention provides a packing adapted for use in conditions involving extreme conditions of heat and pressure without suffering distortion or dislocation while continuing to supply effective mass transfer functions which is comparatively easy to manufacture.